‘Geophagus’ brasiliensis and the other similar-looking species in their group are not true Geophagus and will eventually be given their own genus. These fish are often large-growing, with brasiliensis males reaching up to 10” and having the aggression of Central American cichlids. The exception there is probably ‘Geophagus’ iporangensis, which is essentially like a miniature brasiliensis that tops out at 4 - 5”. Juveniles of brasiliensis and iporangensis are very difficult to tell apart, though, so you often have to grow them out to find out what their adult size will eventually be. ‘G.’ iporangensis are much less common than brasiliensis.
Geophagus sveni are part of the ‘true’ Geophagus — the surinamensis group. These fish are more peaceful (relatively) than the brasiliensis group. They do best in loose shoals and are true to the eartheater name, sifting sand in search of food. These will still reach 6 - 8”.
The last fish you mentioned being available locally sounds like Rio Olimar, which is the collection location for Gymnogeophagus labiatus. These also are not geos, but gymnos. Most of the gymno species are southern SA cichlids coming from temperate rather than tropical climates — southern Brazil, Uruguay, etc.
Unfortunately as adults you probably won’t even have space for a single pair of brasiliensis in a 75 because of their size and aggression. There also isn’t much room for small groups of true geos unless you pick a small species (Geophagus sp. Red Head Tapajos or Geophagus parnaibae, which is rare). G. sveni and similar would probably appreciate a larger tank if you keep them in groups.